In the field of practical lightning protection, there is a wide spectrum of technologies currently being used. To the left of this spectrum there are the air terminals claiming enhanced or more consistent performance. Whether these terminals enhance or retard corona development, or whether they are blunt or sharp, they have been broadly categorized under a generic term "ESE" meaning Early Streamer Emission.
In the center of the spectrum there is the conventional practice widely specified in various Standards. This practice currently uses an electrogeometric model known as the "rolling sphere" which was adapted from the electric power transmission industry, and which is based on no electric field enhancement irrespective of air terminal location or configuration. The rolling sphere is notable for gaining credence from measurements taken on transmission lines. These lines are remarkable for their essentially two dimensional aspect and uniformity of height and conductor diameter. It was from this restricted base that the system was unilaterally adapted into the protection of three dimensional and complex geometrical structures.
Within the Standards there is permitted a widely divergent practice. This may vary from clusters of, for example, six (6) meter high so called Franklin rods to much shorter terminals, sometimes called finials, spaced at closer intervals. There are also the systems with no vertical terminals, sometimes called the Faraday system, and which comprises conductors laid horizontally on exposed surfaces.
To the right of the spectrum are the systems that claim absolutely to prevent lightning attachment by the use of arrays of sharp points designed to produce abundant corona. The corona is claimed to weaken the strength of the near electric field and cause the lightning to strike elsewhere.
While none of the above techniques offer perfection, there is room to improve performance of air terminals and their location through better understanding of the attachment process.
There are four phases in the attachment process of lightning to a ground point. The first is the quasi-static phase where electrical fields build below a storm cloud over some tens of minutes. These fields cause ground objects to be electrically stressed and, dependent on their height and geometric shape, they will emit corona. In the case of a negative cloud base, this corona is in the form of positive ions which create a space charge in the near field immediately above the object.
In the longer term, these positive ions, which in reality are molecules of air, ascend with typical velocities of 1 ms.sup.-1 in the fields of 10 kVm.sup.-1 and create non-linearity in the field to heights of several hundred meters. Thus, the electric field strength observed at ground becomes modified before any dynamic event occurs with typical values of 50 kVm.sup.-1 having been recorded as reducing to values below 5 kVm.sup.-1 near ground.
The second phase relates to the approach of a down leader, a filament discharge with average velocity of 10.sup.5 ms.sup.-1 but with 20-50 .mu.s steps or pauses. The inter-pause velocities can exceed 10.sup.6 ms.sup.-1. This conveyance of charge toward ground causes a rapid increase in the field strength observed by ground points. There is very small initial change in the ground observed electric field strength when the leader is at high altitude, but with near approach, values will be escalating at a typical rate of 10.sup.9 Vm.sup.-1 s.sup.-1.
The third phase is when electric field strength observed by a ground point reaches the critical value to create avalanche breakdown. This commences with an initial corona burst in which streamers can develop, one of which may finally develop into a propagating leader. At this time, factors can be dimensioned such as electric field intensification arising from height and ground electrode curvature. Streamer development fields can also be determined in the laboratory, but up to now the laboratory experiments have not been able to readily model the field decay from the surface to "median" values in the first few meters above a terminal. The "median" or "ambient" field is defined as the unperturbed electric field, i.e., that which would exist in the absence of the object. There is a minimum value of the median field required to convert a streamer into a propagating up leader.
The fourth phase is the continuing propagation of the up leader. Once the root of an up leader is formed, it requires the electric field strength ahead of it to exceed 300-500 kVm.sup.-1 to gain the necessary energy to continue propagation.
Embedded within the above four phases is another spectrum based on the strength of electric field to cause breakdown of air, the electric field strength required to cause upward emission of filamentary streamer type discharges, and a value of electric field strength required to convert the filamentary discharge into an up leader. The former value is commonly quoted as having a nominal value of 3 MVm.sup.-1, while the latter value falls within the range 300-500 kVm.sup.-1. Of course, in nature these values will never be exact.
There is a wide variation in geometric shape of ground points which range from sharp points to flat horizontal surfaces. At one end of the geometric shape spectrum is the so-called pointed Franklin rod. Should this rod produce a field intensification of 1000:1, then 3 MVm.sup.-1 at the tip is reached when the median field is only 3 kVm.sup.-1. No streamer development or propagation is possible in such low median fields but a continuing corona emission will provide an ascending space charge of ionoised air molecules in periods long before the initiation of a down leader.
As the center of this spectrum is approached, the field intensification progressively reduces. The center is reached when, for example, a value of 6:1 is achieved. This center of the spectrum would typically be a "blunt" rod which has a rounded upper surface of a given radius (such that the intensification is 6:1). In this case, the field strength at the tip of the rod reaches a corona emission level of 3 MVm.sup.-1 at the time when the median field reaches the leader propagation level of 500 kVm.sup.-1.
At the other extreme of this spectrum is a flat surface with unity field intensification. Hence, the down leader needs to approach very close to produce 3 MVm.sup.-1 at the surface, but when breakdown with corona emission occurs, propagation would not only be absolutely assured, but would most likely be instantaneous.
This spectrum leads to a number of conclusions, namely, that an elevated sharp point becomes unnecessarily active too early in the process, by producing field-reducing corona along with space charge. This blanket of charge particles lying above the grounded point will act as a shield and prevent the point observing the approach of the down leader. The result is that the down leader must approach much closer in order to force the creation of an up leader. It has been discovered that a rounded surface will provide a better performance by minimizing pre-discharge corona and, by suitable radius or diameter dimension, create streamers only when the near and median field can support their conversion to a leader.
Hereafter, three different types of air terminals will be referred to, viz.: (I) A fully grounded conductor as specified in various Standards, i.e., a Franklin rod which is a long cylindrical conductor with a sharp, conical tip, the shorter finial version, or the rodless system of copper tapes commonly known as the Faraday system. Henceforth, these types of air terminal shall be referred to as "conventional passive". (II) A particular type of air terminal comprising a curved conductor, typically a sphere, placed on a conductive rod. The radius of curvature and overall height of this air terminal is dimensioned according to the method to be described. Hereafter, this type of air terminal shall be referred to as "RFI passive", RFI being the acronym for "reduced field intensification". (II) A particular type of curved surface air terminal comprising one or more insulated components which result in a triggering arc to enhance the initiation of the lightning attachment process; henceforth, this type of air terminal shall be referred to as "RFI triggering".
The present invention and method then relate to: (i) significant improvements of Type I lightning air terminals, viz. the Type II terminals, (ii) certain improvements in the Type III system such as that shown in prior Gumley U.S. Pat. No. 4,760,213, and (iii) to a method of design and application of the Type II & III air terminals. Terminals of the type III seen in such patent are widely sold under the trademark DYNASPHERE.TM. by ERICO Lightning Technologies Pty. Ltd. of Hobart, Tasmania, Australia.
The DYNASPHERE.TM. terminal utilizes a generally spherical or ellipsoidal curved surface electrode which is connected to the grounded central conductor by a high impedance current drain. An annular air gap is provided between the top of the generally spherical surface and the top of the central grounded conductor. Such lightning air terminals have a number of parameters such as the size and shape of the spherical surface, the size of the air gap, the shape of the tip of the central grounded conductor, the height of the terminal above the structure to be protected, and the location of the air terminal on the structure. One primary parameter is known as the "electric field intensification factor" which is derived from the height and curvature of the curved surface electrode. These factors have never before been defined in relation to practical lightning protection systems.
There is accordingly a need for an improved, curved surface, RFI air terminal which will provide a more slowly decaying intensification across the near and median fields, and which creates a trigger or corona only when there is sufficient energy, particularly in the median field, to progress a streamer into a leader. In this way, the field reducing effect and non-linearities associated with corona and space charge are avoided.